[music playing]

Narrator: Most of the time our vision does a wonderful job. But there's a limit to how much our brains can cope with. So we just pay attention to one thing at a time and we're blind to the rest.

Professor: The brain has a huge amount of information being delivered to it by the eyes. It has to make a decision. It has to decide where to place its attention, what information it should scrutinize, what information it should ignore.

Narrator: To test just how much we ignore, we've asked Richard Wiseman and the London Towers basketball team to try out an experiment designed by scientists at Harvard University. The idea is simple but silly. We film the team practicing, and then this happens. Surely no one would miss that. We get a group of people together and Richard briefs them to watch the screen without letting on what they're being tested for.

Professor: So this is an experiment. It's a battle of the sexes, men and women. There'll be a difference, but I'm not going to say which way around it's going to be. And it's just a simple observation test. All you need to do is you'll see there are three guys in yellow here and they have a basketball. And it's your job to count the number of times they throw the basketball to each other.

Now to make things slightly harder, there's also three guys in blue track suits. You ignore them, ignore their basketball, and just concentrate on this one. So if we can run the tape. OK. So that's number one.

Narrator: Crucially, the volunteers are told to count how many times the yellow team passes the ball. But will that be enough to make them miss the gorilla? So did they see anything?

Professor: OK, if we can stop the tape there for the moment. So how many basketball throws? 20? Anymore? Any others? 21? OK. OK, be honest here. Anybody notice anything a little bit unusual? Be honest. OK, it's just about four or five of you. Excellent. The rest of you not notice anything strange? Honest? OK.

Right. For you guys, enjoy this moment. The first time I saw this it completely threw me. Now I want you to watch the tape again, but this time just watch as you would a normal piece of television. No counting the basketball or anything like that. If we can have the tape?

Narrator: Time to show them what they missed.

Professor: Here it comes.

[laughter]

Man: No way.

Man: That didn't happen.

Man: That did not happen.

Woman: No way.

Woman: How did he do it?

Woman: No way.

Man: I didn't even see it.

Woman: No way.

Narrator: Because they were all concentrating on counting, their brains just didn't register the gorilla.

Woman: I'm absolutely shocked. I can't believe it. I was so proud of myself as well because I got all 20. And I was so busy concentrating on it that I didn't— no. Yeah.

Man: I'm kind of embarrassed. I thought I'd spot a monkey walking across the middle of the screen.

Man: I was completely astounded. It was just kind of like, when he did this kind of—

[pounding chest]

—like that, I was just absolutely amazed. I was gobsmacked, I would say. And saying that they might have probably switched the video or something like that. I can't believe that you could miss something that obvious.

Woman: That's really bizarre. It's freaked me out a bit.